I'd say it's indirectly addressing the upset fans and confused viewers of the change in actors. Tony's at a Senate hearing about whether or not the Iron Man suit is a weapon and should be handed over to the government. Of course Rhodey, who served as a sort of liaison between the military and Stark would be there. There is no reason Tony should be surprised he makes an appearance, and given that, no reason that Rhodey should have to tell him "It's me, deal with it". However, the audience has every reason to be surprised and/or upset, and that is who the line is for.

That being said, this is far too much energy expended over what was initially a joke about the Mandarin's rank among comic book villains on film, due to his ability to leave people on screen and off in disbelief over his identity.

That's the key word. It needs to be directly addressing the audience in order for it to break the fourth wall. Rhodey needs to literally be aware of the fact that he is a fictional character in a movie, which he isn't, in order for it to be breaking the fourth wall.